Mystique is a dog who lives at Lola ya Bonobo, [the wildlife sanctuary in the Democratic Republic of the Congo] where Vanessa and I study bonobos. During the day, she is sweet and demure, but at night she becomes a different animal. She guards our house, barking ferociously every time someone comes within earshot. Usually in Congo, a little extra security is appreciated. The only problem is that our house is on the main trail where the night staff walk back and forth after dark. Mystique dutifully barks at all passersby whether she has known them for a day or all her life. Eventually, we just learned to sleep through it. But if there was really a cause for concern, like a strange man with a gun, I wonder if Mystique would bark in a way that would alert me that there was something dangerous and different about the person approaching the house.

Dog vocalizations may not sound very sophisticated. Raymond Coppinger pointed out that most dog vocalizations consist of barking, and that barking seems to occur indiscriminately. Coppinger reported on a dog whose duty was to guard free- ranging livestock. The dog barked continuously for seven hours, even though no other dogs were within miles. If barking is communicative, dogs would not bark when no one could hear them. It seemed to Coppinger that the dog was simply relieving some inner state of arousal. The arousal model is that dogs do not have much control over their barking. They are not taking into account their audience, and their barks carry little information other than the emotional state of the barking dog.

Perhaps barking is another by-product of domestication. Unlike dogs, wolves rarely bark. Barks make up as little as 3 percent of wolf vocalizations. Meanwhile, the experimental foxes in Russia [that have been bred to be docile] bark when they see people, while the control foxes do not. Frequent barking when aroused is probably another consequence of selecting against aggression.

However, more recent research indicates that there might be more to barking than we first thought. Dogs have fairly plastic vocal cords, or a “modifiable vocal tract.” Dogs might be able to subtly alter their voices to produce a wide variety of different sounds that could have different meanings. Dogs might even be altering their voices in ways that are clear to other dogs but not to humans. When scientists have taken spectrograms, or pictures, of dog barks, it turns out that not all barks are the same—even from the same dog. Depending on the context, a dog’s barks can vary in timing, pitch, and amplitude. Perhaps they have different meanings.

I know two Australian dogs, Chocolate and Cina, who love to play fetch on the beach. Each throw sends them plunging through the waves, racing for that magic orb of rubber. When Chocolate retrieves the ball, inevitably Cina wrestles the ball from Chocolate’s mouth, even while Chocolate growls loudly. The girls also eat together, but when Cina tries the same trick with Chocolate’s food, the result is very different. A quiet growl from Chocolate warns Cina away.

It is difficult to see how Cina knows when it is okay to take something from Chocolate’s mouth, since both growls are made when Chocolate is aggravated and unwilling to share. If anything, Chocolate’s growl seems louder and scarier when she is playing than when she is eating.

Experiments have now shown that dogs use different barks and growls to communicate different things. In one experiment, researchers recorded a “food growl” where a dog was growling over food, and a “stranger growl” where a dog was growling at the approach of a stranger. The researchers played these different growls to a dog who was approaching a juicy bone. The dogs were more hesitant to approach if they heard the food growl rather than the stranger growl.

In another experiment, researchers recorded “alone barks” of dogs when they were alone, and “stranger barks” when a stranger was approaching. When researchers played three “alone barks” to different dogs, these dogs showed less attention to each bark. But when they played the fourth bark, the “stranger bark,” the dogs quickly jumped to attention. They did the same thing when the barks were reversed, showing that dogs could clearly distinguish between the two types of barks. Using a similar test, the dogs also distinguished between the barks of different dogs.

How well do people understand what dogs are saying? Researchers played a collection of barks to a group of people. Regardless of whether they owned a dog or not, most people could tell from a bark whether a dog was alone or being approached by a stranger, playing or being aggressive. Unlike dogs, people were not very skilled at discriminating between different dogs. The only time people could tell between different dogs was when they heard the “stranger bark.” This is the exact moment a dog owner would be most likely to want to understand the meaning of a dog bark, since strangers can mean trouble.

These initial studies show that growls and barks do carry meaning that other dogs and, in some cases, people can recognize. This complexity comes as a surprise. Of course, our dogs have known all along—just ask Chocolate and Cina. Still, we know very little about the vocal behavior of dogs.

Brian Hare is an associate professor of evolutionary anthropology at Duke University and the Center for Cognitive Neuroscience, where he founded the Duke Canine Cognition Center. Vanessa Woods is a research scientist at the center as well as an award-winning journalist and the author of Bonobo Handshake: A Memoir of Love and Adventure in the Congo (Gotham, 2010). Hare and Woods are married and live in North Carolina.